1. Field of the Invention
The present invention relates to a two-wavelength optical pickup device that is used for recording and reproducing data on optical recording media such as CDs (compact discs), DVDs (digital versatile discs) and the like. More particularly, the present invention relates to a step-like diffraction element that is used for matching optical axes of two lights having different wavelengths that are emitted from light sources located at different emission points in the two-wavelength optical pickup device, and a method for designing such a step-like diffraction element.
2. Description of Related Art
CDs and DVDs, which are mutually different in their substrate thickness and recording density, are known as optical recording media. When recording or reproduction of data is conducted with respect to those optical recording media, laser beams of different wavelengths may be required. For example, while a laser beam with a wavelength of 650 nm is required to reproduce data on a DVD, a laser beam with a wavelength of 780 nm is required to reproduce and record data on a CD-R.
A so-called two-wavelength optical pickup device is known as an optical pickup device that reads DVDs and reads and records on CD-Rs. The two-wavelength optical pickup device has a laser beam light source for emitting a laser beam with a wavelength of 650 nm and a laser beam light source for emitting a laser beam with a wavelength of 780 nm mounted as a single light source.
For example, a conventional two-wavelength optical pickup device uses a common optical system for different laser beams in order to make the device smaller and more compact. For this purpose, one of the laser beams that are emitted from laser beam light sources at different emission points is deflected by a diffraction grating to thereby conduct both of the laser beams into a common optical path, and converge them on an optical recording medium through a common objective lens.
The diffraction grating, which is used as a phase diffraction element, is formed from a transparent substrate in which one of its opposing incident face and emission face has a grating surface provided with gratings having protrusions and grooves. The depth of the grating is set such that the diffraction grating phase is 2π, in other words, is set at a light-path difference corresponding to one wavelength, for the laser beam from the laser beam light source with a shorter wavelength. Accordingly, the laser beam from the laser beam light source with the shorter wavelength travels straight path without being affected by the diffraction action, and the laser beam from the laser beam light source with the longer wavelength receives the diffraction action, and its first-order diffraction light obtained by the diffraction is introduced in the common optical path.
When the wavelength of the laser beam light source with the shorter wavelength that does not receive the diffraction action is 650 nm, and the wavelength of the laser beam light source with the longer wavelength that receives the diffraction action is 780 nm, and when the refractive index of the grating material is N and the depth of the grating groove is d, the diffraction grating is composed in a manner to satisfy N×d=650 nm.
In the mean time, a phase difference with respect to the beam of a wavelength 780 is given by the following expression (1).N×d/780×2×π=0.833×2π  (1)
Also the diffraction efficiency of (±) first order diffraction light with a wavelength of 780 nm is given by the following expression (2).(2/π)2×sin2(0.833×2π/2)=0.10  (2)
It is understood from the above that the conventional two-wavelength optical pickup device has a small utilization efficiency of 0.1 for the laser beam with a wavelength of 780 nm that is diffracted by the diffraction grating, in other words, by a phase diffraction element. Therefore, in order to perform recording on CD-Rs, a laser beam light source that is capable of emitting a light amount ten times greater than the ordinary amount required. In order to prevent the light diffraction efficiency from lowering, a step-like diffraction element in which its grating is configured in a step-like manner needs to be used as a phase diffraction element.
Also, when the light amount is modulated, the wavelength of a semiconductor laser that is used as a light source varies by several nm depending on the light amount. As a result, the diffraction angle changes due to the changes in the wavelength, which causes a deviation in the optical axis. A deviation angle Δθ of the optical axis is given by the following formula (3).Δθ=sin−1(Δλ/P)  (3)
where, Δλ is an amount of the change in the wavelength at the time of reproducing and recording data, and P is a grating interval.
Occurrence of deviations in the optical axis at the time of recording is not appreciated because this may cause problems such as tracking deviations or deformations in pit configurations.
On the other hand, the optical system that uses two wavelengths suffers chromatic aberrations in which, for example, focal distances of the objective lens and the collimator lens become different depending on the wavelengths. For this reason, the position of light advancing direction of a light emission point of each light source must be decided according to the chromatic aberration of the collimator lens. Therefore, the design of a two-wavelength light source is dependent on the design of a collimator lens and therefore hardly has any degree of freedom.